1. Field of the Invention
The present invention relates to a projection lens, in particular, a projection lens for a projector that projects an image in a spatial optical modulator onto a screen in a larger size.
2. Description of the Prior Art
The following method conventionally is known to obtain large screen images. An optical image corresponding to an image signal is formed in a spatial optical modulator, and the optical image is irradiated with light so that the image is projected in a larger size onto a screen with a projection lens. Recently, a projector using a liquid crystal panel as the spatial optical modulator has been receiving attention. A projector using a reflection type liquid crystal panel as the spatial optical modulator can direct illumination light from a light source toward a projection lens more effectively.
As a method for obtaining color images, the following methods are known: a method where three liquid crystal panels corresponding to image signals of R (red), G (green) and B (blue) are used, and a dichroic prism is used to synthesize the images of the three liquid crystal panels; and a method where light from a light source is divided into R, G, and B based on the time.
The projector using a prism requires a large space between the lens and the spatial optical modulator, so that the projection lens is required to have a long back focal length.
The projector using a prism has large dependence on the angle of incidence from the prism. Therefore, the transmittance of a light ray that is incident at an angle other than a designated angle of incidence is changed, thus resulting in non-uniformity in color and brightness. Therefore, the projection lens requires telecentric properties.
In a projector for data display or graphics display that strictly requires high performance at the periphery of a screen, it is important to eliminate the distortion of the outlines of graphics or blur of colors. To meet this end, it is necessary to correct the distortion aberration and the magnification chromatic aberration sufficiently.
Brightness is required at the periphery, so that the light quantity ratio at the periphery is required to be 70% or more.
Japanese Laid-Open Patent Publication (Tbkkai-Hei) No. 2-228620 discloses a projection lens having a long back focal length and telecentric properties, where distortion aberrations are corrected. However, the following problem is caused.
The projection lens shown in the examples in this publication has a small half angle of view of 36.degree., so that the size of the set is too large to be used as a projection television set.